The CSI is Reliable for Plaster-based Pools

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Working with the results of experiment 1:

-0.7 CSI over 6 months
45 ppm CH increase
4.5 gallons of water
44 sq-in of surface area

According to this source: https://www.hubermaterials.com/userfiles/files/PFDocs/DuraWhite%20Marble%20Aggregate%20for%20White%20Pool%20Plaster%20Applications.pdf

Their aggregate has a solid density of around 169 lbs/ft3 and is about 96% calcium carbonate.

[EDIT]The mix with portland cement per onBalance below is about 60% aggregate and the mix would have a density of 140 lbs/ft3. The CSI is Reliable for Plaster-based Pools - Page 2

So 45 ppm in 4.5 gallons of water is about 0.0028 lbs of plaster or 0.035 in3 of plaster. Spread over the coupon surface results in about 0.0008" of surface plaster which makes the dissolution rate about 0.0016" per year. That assumes of course a constant CSI over that time period. Over 20 years that would be only 0.032". Even a CSI of -1 (2X) would result in only 0.064" over 20 years. Hardly something to get overly concerned about.

Experiment #2 is a bit more difficult because CSI was allowed to drift but if one uses an average PH of 2.37, you would come up with the same dissolution rate modified by the PH change (46x). So that is encouraging that the two results are at least consistent.

I think it might be worthwhile to repeat a test to confirm these results.
 
Thank you for running that math. And I like that you conclude that there is some consistency with the CSI.
I agree that a CSI of -0.7 is not going to cause a significant plaster problem in just one year, or even 3 or 4 years. And that is one of the points of this thread, and I am glad you point that out. It is unfortunate that our plastering industry has been able to fool many others regarding this.

I do have one catch on those numbers. The link is to an "aggregate" material company. Marble aggregate (which is calcium carbonate) comprises about 60% of pool plaster, with Portland cement being the other 40%. The hydration of Portland cement produces mostly Tri and Di calcium silicate hydrates (about 70-75%), with a small amount of Tricalcium aluminate, and about 25% calcium hydroxide. Most of the calcium hydroxide (at surface) is converted into calcium carbonate by balanced water, which is a weaker (more soluble) component than the silicates and aluminates. I believe that needs to be taken into account.
 
Just to confirm, are you saying that the calcium carbonate is 60% + 40% * 25% = 70% of the total weight of the composite material?

Also, it looks like Portland cement is a bit less dense than the CC at 94 lbs/ft3.
 
I would say that the matrix of common pool plaster is about 60% calcium carbonate (the aggregate portion), 30% (primarily) calcium silicates, and 10% calcium hydroxide (the cement portion). Only a very thin layer at the surface (about 3-6 micro-meters??) is calcium carbonate (calcium hydroxide is converted to calcium carbonate).
It is unfortunate that the composition of pool plaster isn't consistent and straight-forward.

Portland cement definitely has a lower density than does the (calcium carbonate or limestone) marble aggregate.
 
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